The use of wireless telecommunications has undergone substantial growth in recent years and is projected to continue expanding as service improves and new products and features are offered. To retain existing customers and entice others to adopt wireless telecommunications, however, services must be provided at a reasonable price. Therefore, the cost of providing wireless telecommunication services must be reduced.
Conventional wireless systems provide service to geographical areas divided into circular or hexagonal cells; the division of the service provides a reason why such systems are commonly referred to as "cellular" systems. The number and size of these cells are selected by the service provider such that geographical coverage is optimized, cost is reduced, and capacity within the service area is maximized. Each cell is equipped with transmitters, receivers and antennas located at a cell site that is typically located near the geographical center of the cell. Each cell site within a particular service area is connected to a central office that serves as a mobile switching center ("MSC") and which controls mobile operation within the cells. The MTSO routes calls to and from other mobile units and the public switched telephone network ("PSTN").
As a practical matter, cell boundaries are not precise. The conventional hexagonal cell shape was chosen because it provides a practical way of covering an area without the gaps and overlaps in coverage that would occur if circular cells were used. Although circular cells could be serviced by omni-directional antennas, directional antennas are often used to reduce the number of required cells by increasing the coverage range for each cell, or to increase traffic capacity by decreasing inter-cell interference. Thus, considerable planning is needed to define the coverage area of each individual cell while minimizing the need to realign cells in the future.
Present cellular systems typically use 120.degree. or 360.degree. antennas mounted on a tower at each cell site. Because these antennas cover a wide angle, their use may limit either cell coverage due to low gain or system capacity due to high levels of interference. Therefore, more cells must be used to adequately service a geographical area and/or traffic load.
To maximize the geographical area and/or reduce inter-cell interference for each cellular site, it is desirable to use high-gain, directional antennas. These antennas are capable of covering a much greater distance than the low-gain, omni-directional antennas currently used by cellular systems. Thus, only one cell site would be required to cover the same geographical area that requires several cell-sites using typical low-gain, omni-directional antennas.
High-gain, directional antennas include multiple elements that can be excited by a drive signal at different power levels or phase angles to tailor the shape or direction of the antenna beam. In order to use such antennas for cellular service, it is necessary to house the antennas in a single structure atop an antenna tower. The required structure, however, presents a high wind load. Typical antenna towers used for cellular antenna sites are not perfectly rigid and are susceptible to sway, or structural bending, due to the wind load. If the antenna tower sways, the portion of the antenna tower proximate the active antenna is effectively rotated about its nominal position and the antenna beam direction is shifted. The shift of the antenna beam direction is not a problem for low-gain, omnidirectional antennas because of the relatively broad, short-range beam that characterizes such antennas. However, for a high-gain, directional antenna (having a relatively narrow, long-range beam), the shift would result in an erratic coverage area.
One possible solution to the problem of using high-gain, directional antennas for wireless communications systems is to use a more rigid antenna tower that is resistant to wind loads. This solution may defeat a principle advantage of using high-gain, directional antennas (i.e., lower cost system). The use of high-gain, directional antennas requires less cell sites to cover a given geographical area than if low-gain, omnidirectional antennas are employed. The savings realized from fewer cell sites would be offset by the increased cost necessary to provide more stable antenna structures. Furthermore, more-stable antenna towers might be too large to be located in a desired location.
An alternative to using more stable antenna structures with high-gain, directional antennas is to use a mechanical gimbal system to maintain the proper angular position of the antenna platform mounting atop the structure. U.S. Pat. No. 4,596,989, entitled "Stabilized Antenna System having an Acceleration Displacement Mass, by Smith et al., issued on Jun. 24, 1986, discloses a stabilized platform for use in connection with a satellite antenna mounted to a ship. The system includes an acceleration displaceable mass, and may include in combination a gimbal mounting and one or more gyros. Although this system might be adaptable to cellular antenna structures, such a mechanical system is extremely complex and susceptible to failure. Furthermore, unlike a ship, cell sites are generally unattended and, if a failure does occur, service would be lost for an extended period. Moreover, the mechanical nature of the system requires routine maintenance.
A second possible solution to using a more stable antenna structure with high-gain, directional antennas is to somehow counteract the tendency of less stable structures to bend due to wind loads. U.S. Pat. No. 4,956,947, entitled "Live Tendon System Inhibiting Sway of High Rise Structures and Method, by Middleton, issued on Sep. 18, 1990, discloses a live tendon system for inhibiting sway of high-rise structures. The system employs sensing means to detect a deflection of the structure, and a controller that actuates tension adjusting members coupled to the structure's main support members in response to the deflection. As with the mechanical gimbal system disclosed by Smith et al., a live tendon system is predominantly a mechanical system requiring routine maintenance to avoid failure. Furthermore, both the systems of Smith et al. and Middleton would substantially increase the cost of each cell site thereby diminishing the principle advantage of using high-gain, directional antennas for cellular systems.
Therefore, what is needed in the art is way of counteracting antenna tower sway that takes advantage of the inherent ability of an active antenna to be redirected by means of its electrical drive signal.